Spectrophotometers have already been known which are suitable to determine the transmission or reflection spectrum of different samples at a number of discrete wavelengths. In one of these spectrophotometers filters are turned before a broadband source, e.g. an incandescent lamp, whereby the light of the lamp is passed successively through different filters onto the sample to be tested, and the electronics connected to the detector sensing the light reflected or transmitted by the sample is synchronized with the turning of the filters. In this case the significant heat output of the lamp causes adverse warm up of the equipment. The thermosensitivity of the detectors and the interference filters in this particular case results in inaccuracy of measurements, and a thermal stability can only be attained a long time after switching on the equipment. In case of using interference filters a further measuring error is produced by the fact that, considering the total spectrum of the broadband source of radiation, the amount of the energy transmitted in the non-pass range of the interference filter is not negligible compared to the energy transmitted in the pass band. This is due to the fact that the bandwidth of the non-pass range is much higher than that of the pass-band.